Organic field-effect transistors (FETs) which employ (BEDT-TTF)(TCNQ) films for active layer have been fabricated and characterized. Their FET characteristics exhibited both p-channel and n-channel operation by changing the gate and drain voltages. For a particular bias condition, the I-V curves revealed behavior where both electrons and holes simultaneously are injected from source and drain electrodes. These bipolar type characteristics are strongly related to the structure of donor and acceptor molecular layers. The degree of charge transfer of approximately 0.2 was estimated by Raman spectroscopy.